1. Field of the Invention
The present invention relates to medical diagnostic and treatment methods and apparatus.
2. Discussion of the Related Art
Transcranial magnetic stimulation (TMS) is a technique for stimulating the human brain non-invasively. TMS uses the principle of inductance to get electrical energy across the scalp and skull without the pain of direct percutaneous electrical stimulation. It involves placing a coil of wire on the scalp and passing a powerful and rapidly changing current through it. This produces a magnetic field which passes unimpeded and relatively painlessly through the tissues of the head. The peak strength of the magnetic field is related to the magnitude of the current and the number of turns of wire in the coil. This magnetic field, in turn, induces a much weaker electrical current in the brain. The strength of the induced current is a function of the rate of change of the magnetic field, which is determined by the rate of change of the current in the coil. In order to induce enough current to depolarize neurons in the brain, the current passed through the stimulating coil must start and stop or reverse its direction within a few hundred microseconds.
TMS is currently used in several different forms. In a first form, called single-pulse TMS, a single pulse of magnetic energy is delivered from the coil to the patient. Repetitive TMS or rTMS, refers to the delivery of a train of pulses delivered over a particular time period. An example of rTMS could be a train of pulses having a 10 Hz repetition rate that lasts for approximately 8 to 10 seconds. In a typical application, this train of pulses is repeated every 30 seconds for up to 20 or 30 minutes.
Magnetic resonance imaging (MRI) is a technique for non-invasive imaging and diagnosis of body organs that uses the interaction between a magnetic field and protons in the body to provide images of body tissues. Functional MRI or fMRI is a subset of this technology and produces images of activated brain regions by detecting the indirect effects of neural activity on local blood volume, flow, and oxygen saturation. MRI systems have been commercially available for a number of years.
The inventors have realized that it would be desirable to combine TMS and MRI technologies in order to provide diagnostic and therapeutic benefits.
Conventionally, however, these two technologies have not been combined for a variety of reasons. First, it has been thought that there may be interactions between the TMS equipment and the MRI equipment due to the fact that both types of equipment generate and use magnetic fields. Therefore, the instantaneous magnetic field associated with the discharge of the TMS coil, which may be on the order of more than two TESLA might interact with the 1.5 TESLA static magnetic field of the MRI system in some unpredictable manner. Second, the discharge of the TMS coil near the sensitive imaging coil of the MRI system might disable or destroy the receiving circuitry within the imaging coil. Third, the mere presence of the TMS coil near the patient's head might contribute artifacts into any images provided by the MRI system. Fourth, the TMS electronics alone might produce artifacts on the images produced by the MRI system.